Coating Techniques

1. Reverse Gravure coating

This coating system is based on an engraved roller immersed in a tank, where the coating material fills the engravings or slits of the roller. The coating is deposited on the substrate as it passes between the engraved roller and the pressure roller while excess material is removed by the doctor blade.

Off-set gravure coating is also commonly used. In this case, the coating material is deposited on an intermediate roller before being transferred to the substrate.

 

 

 

 

 

 

 

 

 

 

 

 

 

2. “Reverse Roll” Coating

In this technique the coating material is measured onto the application roller thanks to precision setting of the gap between the metering roller lying above the application roller. The coating material is brushed off the application roller by the substrate as it passes around the bottom support roller.

The diagram shows a  3-roll reverse roll machine, although the 4-roll versions are more common.

 

 

 

 

 

 

3. Immersion (Dip) Coating (A Foulard)

In this process, the substrate is dipped into a bath of the coating material, normally of low viscosity, so that the coating can run back into the bath as the substrate emerges.

This process is frequently used on porous substrates.

 

 

 

 

4. Meyer Bar Coating

In this process, an excess coating is deposited on the substrate by means of a roller immersed in a tank. A threaded steel bar (the Meyer bar) allows the required quantity of coating to remain on the substrate. The quantity is determined by the diameter of the threading on the bar.

This coating system caters for wide tolerances in precision on the machines; this is why it was one of the first systems to be used.

 

 

5. Curtain coating

Here the coating material, contained in a tank, filters through a gap onto the substrate running underneath. Hence the quantity deposited depends on the speed of the substrate and width of the gap.

 

 

 

 

 

 

 

 

 

 

6. Slot Die Coating

In the so-called Slot Die process, the coating is squeezed via a slot onto the substrate. When the coating is 100% solid, the process is called extrusion. In this case the line speed is faster than the speed of extrusion. This enables coatings to be deposited that are considerably thinner than the width of the slot.

 

 

 

 

 

7. Knife Over Roll Coating

With this type of machine, the coating material is first deposited on the substrate, which then  passes between the knife and support roll, and lastly is freed from  the excess. This process can be used for high viscosity products to obtain high coat weights as in the case of PVC plastisols or resin-coating of textiles. There are several variations on this relatively simple process which is rather basic and therefore somewhat inaccurate.

 

 

8. Air Knife coating

This is a simple process where the coating is applied to the substrate and the excess blow off by a power jet of air. This process is typically used for water-based products and is particularly noisy.